Electromagnetic apparatus for producing linear motion

ABSTRACT

Electromagnetic apparatus for producing an axially directed force comprising a stator having a plurality of axially spaced coils around a working volume within which an armature in the form of a free piston is axially displaceable, the armature having means for producing a magnetic field a substantially radially directed part of which intersects at least some of the coils of the stator such that a resultant axially directed force is generated when a current flows in the coils.

The present invention relates to electromagnetic apparatus for producinglinear motion. A number of designs of linear electromagnetic actuator,sometimes termed a linear motor, have been produced. Various linearmotor configurations are described in WO93/01646. The devices to whichthis prior art relates have axial symmetry and are formed aspiston-in-cylinder machines. The principal advantage of axial symmetrylies in the fact that the strong attractive forces between the magneticelements of the stator and the magnetic elements of the armature arebalanced about a central axis, so that the bearings of the machine donot need to withstand attractive forces.

A further advantage of the axially-symmetric construction is that themagnetic fields of the machine, whether generated as a result ofelectrical currents in conductors or produced by permanent magnets, canbe contained within an outer steel case of the actuator. However theyare generated, the magnetic fields can intersect the electrical coils ofthe machine with a high degree of efficiency.

Yet another advantage of the axially-symmetric construction is that, byutilising a cylindrical stator it is possible to form a sliding sealbetween the stator and the armature, which then allows of thepossibility of creating a gas spring between the stator and armature byclosing one end of the stator. Such an arrangement is described in moredetail in the Applicant's international application PCT/GB98/02823.

All these known linear actuators are constructed and designed to apply aforce between machine components connected or mechanically linked to thestator and armature, for which purpose these have a configuration whichpermits such connection. However, there are certain mechanical systemsin which it is not required to connect a moving element physically to adrive element. In such circumstances the armature of the actuator doesnot have to have a continuous direct connection to the externalenvironment. For example, it may be required for an element to berepeatedly driven to reciprocate along a guide member so as to deliverenergy at the ends of its stroke when it changes direction. A roadbreaker or so-called “pneumatic hammer” works in this way by thealternate application of opposite forces by directing pressure fluidalternately into opposite chambers of a pneumatic cylinder with aflutter valve.

If the moving element has significant inertia, against which theexternal “stator” of the machine is designed to experience a reaction, acontrolled vibration can be applied: this may be used for example inassociation with earth-moving machinery. As a third example, the motionof the an unrestricted piston might be used to vary the total enclosedvolume and/or pressure of a fluid system to which the piston chamber isconnected. Such a machine might be used as a fluid pump, to dispensemetered fluid, to vary the pressure in a sensory pad or to form part ofa sensitive and precise fluid pressure suspension system.

Examples of such devices are disclosed in patent specifications U.S.Pat. No. 4,965,864, GB 6 452 81 and GB 2 017 420.

Specification U.S. Pat. No. 4,965,864 discloses a pump in which there isa hollow cylindrical stator surrounded by a sequence of coils and anarmature which acts as the pump piston. The armature is a series ofmagnets arranged to produce a succession of radially oriented magneticfields of the same sense. The coils are energised sequentially to causethe armature to oscillate within the stator and perform the pumpingaction.

Specification GB 6 452 81 discloses a solenoid device which works on thevariable reluctance principle. The device consists of a plurality ofsequential coils surrounding a hollow cylindrical chamber. A magneticarmature has an axial length such that it overlaps two adjacent coils.Successive energising of the two adjacent coils overlapped by thearmature causes a linear motion of the armature, either in a singledirection or

Specification GB 6 452 81 discloses a solenoid device which works on thevariable reluctance principle. The device consists of a plurality ofsequential coils surrounding a hollow cylindrical chamber. A magneticarmature has an axial length such that it overlaps two adjacent coils.Successive energising of the two adjacent coils overlapped by thearmature causes a linear motion of the armature, either in a singledirection or in a reciprocating motion, as desired.

Specification GB 2 017 420 again discloses a electromagnetic pump whichhas an armature which acts as an piston and which is caused toreciprocate within a stator chamber by electromagnetic interactionbetween the armature and a winding surrounding the stator and whichproduces a reciprocating magnetic field which causes correspondingmovement in the armature.

In this case the armature is made of an electrically conducting butnon-magnetic material such as aluminum and the device acts,inefficiently, as a reciprocating linear induction motor.

The present invention seeks to provide a linear actuator comprising astator and an armature in which the armature is not attached to anoutput member which protrudes from the stator, such as the actuator armor piston rod of known such actuators. That is to say, the armature ofthis invention is a piston that is free (or substantially free) to movewithin the stator volume. As used in this specification the term “freepiston” will be understood to mean a member movable within a workingvolume without having a force-transmitting member such as a piston rod.

According to one aspect of the present invention, therefore, the presentinvention provides electromagnetic apparatus for producing an axiallydirected force comprising a stator having a plurality of axially spacedcoils around a working volume within which an armature in the form of afree piston is axially displaceable, the armature having means forproducing a magnetic field a substantially radially directed part ofwhich intersects at least some of the coils of the stator such that aresultant axially directed force is generated when a current flows inthe coils.

Preferably the physical parameters of the device are so chosen that theelectrical coils may be connected to an electronic drive unit forcontrolling the phase, frequency or amplitude of the current in thecoils so as to cause the desired axially-directed electromagnetic forceto be created between the armature and the stator.

In one embodiment the armature and the stator are of circular crosssection. This makes it easy for the outer surface of the armature to besealed with respect to the internal surface of the stator, so as toprevent or at least restrict fluid flow between the volumes on eitherside of the piston.

Means may be provided for sensing the position of the piston so as tooptimise the configuration of the currents supplied to the coils by theelectronic drive unit.

The electronic drive unit may also be arranged to produce a signalrepresentative of the current supplied to the actuator, the integral ofsuch signal being used to control the pressure of a gas supply to atleast to one side of a sealed armature.

Various embodiments of the present invention will now be moreparticularly described, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic axial sectional view of a free piston actuatorformed as a first embodiment of the present invention;

FIG. 2 is an axial sectional view of an alternative embodiment havinginduction drive and a working tool;

FIG. 3 is a schematic axial sectional view of an alternative embodimentutilised in a pressure pad drive system; and

FIG. 4 is an axial sectional view of a further alternative embodimentemploying ring magnets.

FIG. 1 shows a particular form of the device in which thearmature(piston) is required to be sealed to the inner surface of thearmature tube. In this example the stator 1 carries an assembly of coils2 extending circumferentially around the axis of the cylinder. Thepiston 3, moving on bearings 9 and seals 6, carries an arrangement ofmagnets 4 and polepieces 5 to produce a magnetic field alternating inpolarity along the axis of the cylinder. The ends of the device includeclosures 7, in which, in this embodiment, there are provided fluidpassages 8 so as to allow fluid to be moved by the action of the piston3, if that motion is significant in relation to the enclosed volume.

FIG. 2 shows an example of a free-piston actuator used to deliverrepeated blows to a tool 10. The armature of this device consists of ahardened core 11 which may also contain lead to increase its mass. Thecore is keyed to an outer magnetic steel cylinder 12 which carries aseries of slots in which are copper rings 13.

The coil assembly 2 is energised to produce an alternating magneticfield that travels along the axis of the piston. This induces currentsin the armature rings 13 so as to produce a second alternating magneticfield, whose interaction with the first field accelerates the piston. Itis necessary for the piston to be fitted with simple bearing rings (notshown, but similar to those of 9 in FIG. 1). Nevertheless, it should benoted that the inductive forces that accelerate the piston also act tohold it away from the sides of the cylinder when in motion; suchbearings do not therefore need to be of high quality.

Arrangements (not shown) are made to allow air or other gas within thecylinder 12 to pass freely between the opposing ends of the device shownin FIG. 2, so as not to impede the motion of the piston.

It will be understood that the electromagnetic configuration of theinvention is not restricted to the use of permanent magnets and anironless stator assembly, as shown by way of example in FIG. 1, or tothe use of an induction technique as shown by way of example in FIG. 2.The armature or piston may, with advantage in certain conditions, use avariable-reluctance drive technique and/or the stator may have slotshousing the coils.

FIG. 3 shows a particular form of the actuator in relation to a pressurepad 15 carrying a load 19. The principal parts of the actuator deviceare similar to those described in FIG. 1.

In this example the piston is shown as being provided with bearings 9but to be sealed to the pressure pad 15 and to the source ofpressurising fluid by bellows 18.

FIG. 4 shows a modified form of the device illustrated in FIG. 3, inthat the axially-alternate magnetic field of the piston is produced byplane rings of magnetic material instead of discs. This technique allowsthe device to be of lightweight construction whilst applying force to adiaphragm 20 having an area larger than that of the magnets.

As described in the Applicant's International Patent ApplicationPCT/GB98/02823 the time integral of the current consumed by the deviceis continuously computed and the fluid pressure in the volume 17 beneaththe piston is frequently adjusted to bring this integral to zero. Thiscontrol technique ensures that the mean values of the pressures aboveand below the piston are equalised.

Position-sensing means (not shown) are also utilised by the controlsystem to ensure that the median position of the piston is in thecentral region of the actuator.

It will be understood that the use of a bellows to replace the pistonseal removes the constraint that the piston and the internal surface ofthe armature should be of circular cross-section.

It will be further understood that the principles of this invention arenot compromised by the use of springs or equivalent compliances toprovide a mechanical bias to the position of the piston in the absenceof electrical power and that such compliant biasing devices may utilisepermanent magnetic fields. In embodiments in which the armature isintended, in use, to reciprocate at high frequency the air trapped inthe stator cylinder on either side of the pistons may act as a gasspring to cushion the impact at each end of its stroke. Thepiston/cylinder seal does not have to be very airtight in that theperiodic motion may have a sufficiently high frequency for leakageeffect to be minimal.

What is claimed is:
 1. An electromagnetic apparatus comprising: astator; said stator having closed ends defining a working volume; aplurality of coils in said stator; a free piston axially movable withinsaid stator; means for applying an electrical current to said pluralityof coils in a manner effective for oscillating said piston within saidworking volume; said piston including at least one unitary annularconductive member; and said electrical currents being effective toinduce magnetic fields in said conductive member in a sense effective tointeract with magnetic fields of said plurality of coils thereby togenerate a resultant axial force which urges said piston in axial motionin said stator.
 2. An apparatus according to claim 1, wherein saidarmature includes a plurality of unitary annular conductors attached toa common ferromagnetic body.
 3. An apparatus according to claim 1,wherein: said working volume includes at least one end stop in saidworking volume; said axial motion includes sufficient motion to impactsaid end stop, whereby a kinetic energy of said piston is deliverable toan external object.
 4. An apparatus according to claim 3, furthercomprising: a working tool associated with said stator; and said workingtool receiving said kinetic energy for delivery to said external object.5. An apparatus according to claim 1, wherein: said working volumeincludes at least one opening; said at least one opening permitting atransfer of fluid and from one side to another thereof upon displacementof said piston within said working volume.
 6. An apparatus according toclaim 1, further comprising: a flexible member enclosing said workingvolume to form a closed volume having variable dimensions; means forcommunicating a fluid within said closed volume with a second workingvolume outside said stator; a pressure pad in physical contact with saidsecond working volume; and said pressure pad being effective for varyinga pressure exerted over at least a limited part of an area of saidsecond working volume.
 7. An electromagnetic apparatus comprising: astator; said stator having closed ends defining a working volume; aplurality of coils in said stator; a free piston axially movable withinsaid stator; means for applying an electrical current to said pluralityof coils in a manner effective for oscillating said piston within saidworking volume; said piston including a plurality of permanent magnets;said electrical currents being effective to induce magnetic fields whichinteract with magnetic fields of said plurality of magnets in a senseeffective to generate a resultant axial force which urges said piston inaxial motion in said stator; said plurality of permanent magnets beingoriented with their direction of magnetization parallel to an axis ofsaid stator; and pole pieces at each end of said piston directingmagnetic fields produced by said magnets radially from circumferentialfaces of said pole pieces whereby to intersect said coils.
 8. Apparatusaccording to claim 7, wherein directions of magnetization of adjacentmagnets is in opposite directions.